The present invention relates to a method for the fluidized bed combustion of a fuel in a circulating fluidized bed system and particularly to a method for controlling the extraction of heat from the recycle solids to control the temperature of a fluid such as reheated steam.
Fluidized bed combustion has gained favor for a number of reasons. An outstanding feature is its ability to burn high-sulfur fuels in an environmentally acceptable manner without the use of flue-gas scrubbers. In fluidized-bed combustion, much of the sulfur contained in the fuel is removed during combustion by a sorbent material in the fluid bed, usually limestone. In this process, the production of nitrogen oxides is low because of the low temperature at which the combustion reaction takes place.
One type of fluidized bed combustion is the circulating fluidized bed system. In this system, the gas velocities in the furnace are three to four times as high as in a conventional bubbling fluidized bed system. The small solid particles are carried up through the furnace and a uniform lower-density gas/solids mixture exists throughout the entire furnace. Since the solids move through the furnace at much lower velocities than the gas, significant solids residence times are obtained. The long residence time coupled with the small particle size produce high combustion efficiency and high sulfur oxide removal with lower sorbent limestone feed.
In the circulating fluidized bed combustion system, the solids which are carried from the furnace are separated from the gas by a cyclone. The solids discharged from the bottom of the cyclone pass through a seal pot or syphon seal with a portion of the solids going to a solids heat reinjected directly back into the furnace. The heat extracted from the solids in the heat recovery fluid bed system may be used to provide additional evaporation, superheat and/or reheat.
In order to prevent excessive moisture from forming in the low pressure steam turbine stages, it is conventional to interrupt the expansion process, remove the steam for reheating at constant pressure, and return it to the low pressure turbine stages. This is known as a reheat cycle. In a circulating fluidized bed system, this reheat may be performed in the convection pass of the furnace, in the heat recovery fluid bed system or a combination of these. When the heat recovery fluid bed system is used for reheat, either alone or in combination with reheat in the convection pass, a problem has been encountered in controlling the reheater steam temperature. The conventional way of controlling this reheater steam temperature is by controlling the solids flow from the bottom of the cyclone to the heat recovery fluid bed system containing the reheater. Available means for controlling such solids flow are just not accurate enough to maintain precise temperature control. It is this problem that is addressed by the present invention.